Compositions of polyphenylene ether resins are useful in molding, extrusions and the like. Such resins have repeating structural units conforming to the formula: ##STR1## wherein the oxygen ether atom of one unit is connected to the benzene nucleus of the next adjoining unit; n is a positive integer equal to at least 50; and each of Q and Q is a monovalent substitutent selected from hydrogen, halogen, hydrocarbon radicals free of a tertiary-alpha-carbon atom, halohydrocarbon radicals having at least two carbon atoms between the halogen atom and phenol nucleus and being free of a tertiary-alpha-carbon atom, hydrocarbonoxy radicals being free of a tertiary-alpha-carbon atom, and halohydrocarbonoxy radicals having at least two carbon atoms between the halogen atom and phenyl nucleus and being free of tertiary-alpha-carbon atom. Examples of polyphenylene ethers conforming to the formula and methods for their preparation may be found in Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875, and Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358, each of which is incorporated herein by reference. Compositions comprising polyphenylene ether resins and styrene resins, including styrene/butadiene graft and random copolymers, are disclosed in Cizek, U.S. Pat. No. 3,383,435, which is also incorporated herein by reference.
It is well known in the art that plastic compositions may be rendered flame retardant by the addition of various chemicals known as "flame retardants." Flame retardants can be divided into two types: (1) additives which are blended physically with the material and (2) modifiers which are reactive and unite chemically with the plastic. The former include most organic phosphate esters, halogenated hydrocarbons such as chlorinated waxes, antimony oxide, and inorganic salts. The latter group (2) includes chlorendic acid and its derivatives, halogenated phthalic anhydride, phosphorus and halogen-containing polyols, and halogenated phenols. It should be noted, however, that improving flame retardancy is not simply a matter of adding chemicals. Combining flame retardant additives with various polymers not only affects burning characteristics, it frequently changes the color, flexibility, tensile strength, electrical properties, softening point, and moldability characteristics of the plastic. Obtaining adequate flame retardant properties usually requires loading the plastic with a high concentration of flame retardant.
Prior art flame retardants, such as those noted above, have been added to blends of the polyphenylene ethers, alone, and in combination with styrene resins with some or even substantial improvement in flame retardant properties. For example, aromatic phosphates such as triphenyl phosphates have been added to compositions comprising polyphenylene ether resins with flame retardant properties being improved to the point where they may be classified as self-extinguishing and non-dripping according to the small-scale tests, such as ASTM test method D 635 and Underwriters' Laboratory Bulletin No. 94. However, triphenyl phosphate is a plasticizer for the polyphenylene ether resin and its addition even in the small amounts necessary for flame retardancy, results in a substantial decrease in heat distortion temperature to a point where the commercial use is substantially impaired. Other aromatic phosphates behave in similar manner. Aromatic halogen compounds also have been compounded into polyphenylene ether resins but their addition in commercially feasible quantities generally does not result in satisfactory flame retardant properties, because high loadings lead to discoloration because of processing difficulties.
Haaf, U.S. Pat. No. 3,639,506, incorporated herein by reference discloses that the addition of a small but effective quantity of a flame retardant combination of an aromatic phosphate and an aromatic halogen compound to a composition comprising a polyphenylene ether substantially improves flame retardant properties without depressing heat distortion temperature of the polymer to a point where its commercial use is substantially impaired.
Recently, industry-wide interest in the performance of materials under real-life fire conditions has become increasingly apparent. In contrast to small-scale fire-test performance (e.g., ASTM and Underwriters' Bulletin No. 94), large-scale tests are concerned with parameters such as evaluation of the extent of burning, the heat involved during combustion, smoke characteristics, and the nature of the composition products liberated.
It has now been discovered that a combination of (a) a polyphenylene ether resin, (b) an aromatic phosphate, an aromatic phosphonate or an aromatic phosphine oxide, (c) an aromatic halogen compound and (d) an elastomeric impact modifier comprising a polymerized diene rubber compound performs unexpectedly well during large scale flammability tests, such as the Brady-Williamson Corner Test.
The new combination is uniquely advantageous because (i) it is possible to incorporate a large amount--10 parts by weight, or so--of a brominated fire-retardant additive without causing discoloration or processing difficulties; (ii) peak burning temperatures are low, smoke densities are low and burning, dripping tendencies are reduced; and (iii) large scale and small scale test results are good whereas known compositions with good small test results, fail the large scale tests.